1. Field of the Invention
The present invention relates generally to the field of medical diagnostics. More particularly, the present invention relates to the design and use of catheters for determining physiologic characteristics of body lumens, such as the diameter and wall compliance of blood vessels.
Many bodily diseases and/or abnormalities can be diagnosed by measuring the condition, i.e. size and/or compliance, of body members, one example is the vascular disease atherosclerosis, which involves the narrowing of a blood vessel such as an artery. Such narrowed regions are often referred to as lesions, stenosed regions, and the like. The narrowing of the blood vessel in turn results in the restriction, and in extreme cases, the cessation, of blood flow to the capillary system served by that particular artery. Treatment for atherosclerosis has included both surgical and non-surgical techniques. The surgical techniques involve the removal of the portion of the vessel containing the disease and replacement thereof with a vessel segment taken from another part of the body. Non-surgical techniques involve the use of an intravascular catheter. These techniques include mechanical devices which remove the occluding material in the vessel, laser devices which vaporize the occluding material, and balloon catheter devices which compress the occluding material against the vascular wall.
Heretofore, atherosclerosis has usually been diagnosed by inserting a catheter into the vessel of interest and then injecting a contrast agent into the vessel through the catheter. The blood flow will carry the contrast agent along the vessel so that the vessel can be radiographically imaged with a display device such as a fluoroscope. The radiographic image of the vessel is then reviewed in order to estimate the internal diameter of the vessel to determine if there is any abnormal narrowing of the vessel which may have occurred due to disease. If any narrowing is observed, the extent (percentage) of narrowing is estimated from the radiographic image by measuring the vessel diameter both at and immediately before the region of narrowing with a ruler, calipers or similar device. Such a measurement is typically not particularly accurate since it relies on discerning an ill-defined boundary in a single plane. Additionally, stenotic material outside of the image plane can be missed. These results in average errors of approximately 30%. Such inaccuracy hinders adequate characterization of vascular disease.
Another significant vascular condition known as "hardening of the arteries" typically occurs with aging and is characterized by the vessel wall becoming rigid, resulting in a lost capacity to expand and contract during the cardiac cycle. Normally, the vessel wall is sufficiently compliant that it expands as blood pressure rises and contracts as blood pressure falls within each cardiac cycle. It would be quite useful to accurately measure the compliance of vessel walls to determine the location and extent of non-compliant portions of the vascular system. It would be particularly useful to make such a determination prior to any interventional therapy, such as balloon angioplasty, which results in physical alteration of the atheroma and/or, blood vessel wall. Such early determinations would be of great value in selecting the mode of interventional therapy, best suited for the patient's condition.
The accurate measurement of both size and compliance of other body members, including the intestines, the urethra, and the cervix, among others, would also assist in the determination of particular conditions and/or the diagnosis of disease in those members.
Hence, there is a significant need for a system capable of accurately and directly determining in vivo the size and optionally the compliance of blood vessels, as well as other body members. Such a system should be relatively simple to use and relatively inexpensive, to accommodate a single-use strategy.
2. Description of the Background Art
U.S. Pat. No. 4,651,738, describes a method and system for monitoring the pressure-volume relationship in an angioplasty balloon during conventional angioplasty procedures. The pressure-volume relationship may be plotted on a cathode ray tube display, and the shape of the resulting curve may be used to predict the likelihood of success of the angioplasty treatment. The method relies on balloon inflation to high pressures (up to 12 atmospheres), and no suggestion is made that lumen diameter or cross-sectional area should be measured, either before or during the angioplasty treatment. The measurement of the pressure-volume relationship during angioplasty procedures to elucidate the mechanism of lesion dilation is also described in the medical literature. See, for example, Demer et al. (1991) J. Am. Coll. Cardiol. 18:1259-1262; and Hjemdahl-Monsen et al. (1990) J. Am. coll. Cardiol. 16:569-575. The use of computer-enhanced radiographic imaging techniques for determining vascular lumen diameter before and after balloon dilatation procedures is described in serruys et al. (1984) Am J. Cardiol. 54:482-488; and Nichols et al. (1984) Circulation 69:512-522.